1. Field of the Invention
The present invention relates to a sensor element and a gas sensor.
2. Description of the Related Art
To date, gas sensors for detecting the concentrations of specific gases, e.g., NOx, in gases to be measured, such as automobile exhaust gases, are known. For example, PTL 1 and PTL 2 describe gas sensors including long lengths of plate-shaped sensor elements, in which a plurality of oxygen ion-conductive solid electrolyte layers are stacked. The sensor element includes a flow portion, which is disposed inside, of a gas to be measured, an outside pump electrode disposed outside the sensor element, an inside pump electrode and a measurement electrode disposed in the measurement object-gas flow portion, and a reference electrode, to which a reference gas serving as the reference of detection of a specific gas concentration is introduced.
Regarding such a sensor element, pumping in or pumping out of oxygen is performed between the inside of the measurement object-gas flow portion and the outside of the sensor element by a control voltage applied between the outside pump electrode and the inside pump electrode and, thereby, the oxygen concentration in the measurement object-gas flow portion is adjusted. Subsequently, the specific gas in the gas to be measured after adjustment of the oxygen concentration is reduced, and the specific gas concentration in the gas to be measured is detected on the basis of a current which flows between the outside pump electrode and a measurement electrode in accordance with the oxygen concentration after the reduction. In this regard, feedback control is used for adjusting the oxygen concentration in the measurement object-gas flow portion. Specifically, the oxygen concentration in the measurement object-gas flow portion is determined by measuring the electromotive force between the inside pump electrode and the reference electrode. The control voltage is feedback-controlled so as to make the electromotive force constant.